This invention relates to a system that allows traffic signals to be remotely controlled, and more specifically, a method of optically transmitting data from an optical emitter to a detector mounted near an intersection.
Traffic signals have long been used to regulate the flow of traffic at intersections. Generally, traffic signals have relied on timers or vehicle sensors to determine when to change the phase of traffic signal lights, thereby signaling alternating directions of traffic to stop, and others to proceed.
Emergency vehicles, such as police cars, fire trucks and ambulances, are generally permitted to cross an intersection against a traffic signal. Emergency vehicles have typically depended on horns, sirens and flashing lights to alert other drivers approaching the intersection that an emergency vehicle intends to cross the intersection. However, due to hearing impairment, air conditioning, audio systems and other distractions, often the driver of a vehicle approaching an intersection will not be aware of a warning being emitted by an approaching emergency vehicle. This can create a dangerous situation.
This problem was first successfully addressed in U.S. Pat. No. 3,550,078 (Long), which is assigned to the same assignee as the present application. The Long patent discloses an emergency vehicle with an optical emitter, a plurality of photocells mounted near an intersection with each photocell looking down an approach to the intersection, a plurality of amplifiers which produce a signal representative of the distance of the approaching emergency vehicle and a phase selector which processes the signal from the amplifiers and can issue a phase request to a traffic signal controller to preempt a normal traffic signal sequence and give priority to the approaching emergency vehicle.
The Long patent discloses that as an emergency vehicle approaches an intersection, it emits a preemption request comprised of a stream of light pulses occurring at a predetermined repetition rate, such as 10 pulses per second, and with each pulse having a duration of several microseconds. A photocell, which is part of a detector channel, receives the stream of light pulses emitted by the approaching emergency vehicle. An output of the detector channel is processed by the phase selector, which then issues a phase request to a traffic signal controller to change to or hold green the traffic signal light that controls the emergency vehicle's approach to the intersection.
While the system disclosed by Long proved to be a commercial success, it became apparent that the system would have to be provided with better signal discrimination. The system disclosed by Long occasionally suffered false detections that occurred in response to low repetition rate light sources, fluorescent lights, neon signs, mercury vapor lamps and lightning flashes. It was also found that the system did not adequately discriminate between a series of equally spaced light pulses and a series of irregularly spaced light pulses. In addition, the length of time that the pulse request signal remained active after the termination of light pulses was unpredictable and sometimes too short.
U.S. Pat. No. 3,831,039 (Henschel), which is assigned to the same assignee as the present application, improved on the system disclosed in the Long patent by disclosing a more accurate discrimination circuit that imposed stricter requirements on the stream of light pulses received from an emergency vehicle. In the system disclosed by Henschel, the stream of light pulses must have proper pulse separation and continue for a predetermined period of time. Also, once a preemption request is issued to the traffic signal controller, the preemption request signal must remain active for at least a predetermined time period.
As an example, Henschel disclosed an embodiment where individual light pulses must not be separated by more than 120 milliseconds, the stream of light pulses must continue for at least 1.5 seconds and once activated, the phase request signal must remain active for at least 9 seconds. The discrimination circuit disclosed by Henschel provided an improvement over the discrimination circuit disclosed by Long and resulted in fewer incorrect detections.
Although the system originally disclosed by Long contemplated that optical traffic preemption systems would be used for emergency vehicles, such systems began to be used by authorized vehicles that were not emergency vehicles, such as buses and maintenance vehicles. Subsequently, a need arose to prioritize preemption requests originating from different classes of vehicles. For example, if a bus and an ambulance are each equipped with an optical emitter transmitting a preemption request and both are approaching an intersection simultaneously from different streets, the ambulance should be given priority to proceed through the intersection because a human life may be at stake. This need was addressed by U.S. Pat. No. 4,162,477 (Munkberg) which is assigned to the same assignee as the present application.
Munkberg disclosed an optical traffic preemption system wherein vehicles can transmit preemption requests at different priority levels. The optical emitter disclosed by Munkberg can transmit light pulses at a variety of selectable predetermined repetition rates, with the selected repetition rate indicative of a priority level. The discrimination circuit disclosed by Munkberg can discriminate between different classes of vehicles and assign each class a priority level. Systems constructed in accordance with the Munkberg patent have typically defined two priority levels; a low priority level that transmits approximately 10 light pulses per second and a high priority level that transmits approximately 14 light pulses per second.
The discrimination circuit disclosed by Munkberg employs a delay circuit controlled by a timing pulse generator. One discrimination circuit is required for each discrete repetition rate to be detected. A signal derived from detected light pulses is provided to the delay circuit and is delayed for a time interval equal to the period of the repetition rate to be detected. A delayed signal from the delay circuit is compared with the signal provided to the delay circuit. If the two signals have simultaneous pulses, the detected light pulses can be considered to have originated from a valid optical traffic preemption system emitter.
The discrimination circuit disclosed by Munkberg adequately discriminated between preemption priority levels. However, the system required a large number of discrete and dedicated circuits. U.S. Pat. No. 4,734,881 (Klein et al.) disclosed a discrimination circuit based on a microprocessor. The microprocessor used a windowing algorithm to validate that pulses of light had been transmitted from a valid optical traffic preemption system emitter.
In an embodiment disclosed by Klein et al., an optical traffic preemption system has four detector channels connected to input/output circuitry. The input/output circuitry is in turn connected to the microprocessor. Upon receiving a "first" light pulse at a detector channel, the microprocessor enters a lockout interval. During the lockout interval, no light pulse will be recognized at any detector channel. After the lockout interval expires, a window interval is entered which allows the detector channel that initially detected the first light pulse to receive an additional light pulse. The window interval is very brief and is centered around the point in time at which a light pulse from a valid emitter would be expected. If a pulse is detected during the window interval, the light pulses can be considered to have originated from a valid emitter. The lockout interval and the window interval are successively repeated as the discrimination circuit receives and tracks valid light pulses. However, if no pulse is received during a window interval, the discrimination circuit is reset and all detector channels are again able to detect a "first" light pulse.